1. Field of the Invention
This invention relates to an electrophotographic photoconductor comprising an organic photoconductive layer, and a protective layer formed thereon, which comprises carbon or a carbon-based material as its main component.
2. Discussion of Background
Heretofore, the following photoconductors have been generally used for electrophotography: a photoconductor in which a conductive substrate is provided with a photoconductive layer comprising a the main component selenium or its alloy, a photoconductor comprising a photoconductive layer in which an inorganic photoconductive material such as zinc oxide or cadmium sulfate is dispersed in a binder agent, a photoconductor comprising a photoconductive layer which contains polyvinyl-N-carbazole and an organic photoconductive material such as trinitrofluorenone or an azo pigment, and a photoconductor comprising a photoconductive layer made of an amorphous silicon material.
The electrophotography is one of image-reproducing methods. According to this method, a photoconductive layer of a photoconductor is electrically charged in the dark, for instance, by a corona charge, and an optical image is exposed thereto. During exposure to the image, electrical conductivity is produced within the photoconductive layer in the areas where light impinges upon the photoconductive layer. This causes dissipation of the surface charges in these areas due to conduction through the photoconductive layer to a substrate on which the photoconductive layer is formed. The electrostatic latent image thus obtained is developed with charge detective finely-divided particles, a toner, composed of a coloring agent such as a dye or a pigment and a binder agent made of polymeric material. Thus, a visible image can be obtained.
Basic characteristics required for the photoconductor for use in the electrophotography are as follows:
(1) the photoconductor is chargeable with a predetermined surface potential in the dark;
(2) the photoconductor dissipates only a few electric charges in the dark; and
(3) the photoconductor promptly dissipates electric charges when it is illuminated.
Among the previously mentioned photoconductors currently available, organic photoconductors are now attracting attention. This is because organic photoconductors can be produced at low production cost, and cause little air pollution. In addition, the organic materials allow relatively free designing for preparing photoconductors.
In general, a dispersion or a solution prepared by dispersing o dissolving a charge generating material and a charge transporting material in a binder resin is coated onto an electroconductive substrate to form a photoconductive layer, thereby obtaining an organic photoconductor.
The photoconductive layers are classified into two types, a single-layer type and a function-separated type.
A photoconductive layer of the single-layer type fulfills the charge retaining, charge generating and charge transporting functions by itself. A photoconductive layer of the function-separated type, on the other hand, is composed of some layers such as a charge generating layer which generates electric charges, a charge transporting layer which retains charges and transports the electric charges generated by the charge generating layer, and, if necessary, a layer which hinders electric charges to invade from the substrate, or a layer which prevents the substrate from reflecting light.
The organic photoconductors have, as described above, excellent characteristics. However, their surfaces have low hardness, so that they are easily worn away or scratched when they are brought into contact with a developer, a copying paper or a cleaning member of a copying machine in the course of the reproduction process.
The life span of the photoconductor is undoubtedly shortened by the worn or scratched photoconductive layer. This is because the worn photoconductive layer decreases charged potential, and the scratched one produces offensive line images.
In order to eliminate the above disadvantages inherent in the organic photoconductors, a protective layer with which the organic photoconductive layer is covered has been proposed. The protective layer protects the organic photoconductive layer from undergoing mechanical damages, so that the durability of the photoconductor can be improved. Examples are as follows:
A photoconductive layer covered with an organic film as disclosed in Japanese Patent Publication 38-015446, a photoconductive layer covered with a layer made of an inorganic acid compound as disclosed in Japanese Patent Publication 43-014517, a photoconductive layer on which an insulating layer is overlaid through an adhesive layer as disclosed in Japanese Patent Publication 43-027591, and a photoconductive layer on which an a-Si layer, an s-Si:N:H layer or an a-Si:0:H layer is overlaid by means of plasma CVD or photo CVD as disclosed in Japanese Laid-Open Patent Applications 57-179859 and 59-058437.
In addition, intensive studies are now being made into application of a diamond-like carbon film with high hardness to the protective layer. Examples are as follows:
A photoconductive layer covered with a protective layer made of an amorphous carbon or a hardened carbon material as disclosed in Japanese Laid-Open Patent Application 60-249155, a photoconductive layer covered with a protective layer made of a diamond-like carbon as disclosed in Japanese Laid-Open Patent Application 61-255352, a photoconductive layer covered with an insulating layer having high hardness made of a carbon-based material as disclosed in Japanese Laid-Open Patent Application 61-264355, an organic photoconductive layer covered with a protective layer made of a plasma-polymerized film which at least contains atoms of nitrogen, oxygen, a halogen or an alkaline metal as disclosed in Japanese Laid-Open Patent Applications 63-97961, 63-97962, 63-97963 and 63-97964, and an organic photoconductive layer covered with a protective layer made of an amorphous hydrocarbon film formed by a glow discharge, at least containing a chalcogen atom, an atom belonging to the III group, an atom belonging to the IV group or an atom belonging to the V group as disclosed in Japanese Laid-Open Patent Applications 63-220166, 63-220167, 63-220168 and 63-220169.
In the above all proposals, a thin film made of a carbon-based material with high hardness, which is generally referred to as an i-carbon film or a diamond-like carbon film, is formed on the surface of an organic photoconductive layer by means of an ion process such as sputtering, plasma CVD, glow discharge decomposition or photo CVD.
The surface of the organic photoconductive layer can thus be reinforced, and the durability of the photoconductor is successfully improved.
However, in the case where an excessively hard protective layer is formed on an organic photoconductive layer having a low Vickers hardness of approximately 20 Kg/mm.sup.2, sufficient adhesion cannot be obtained between the protective layer and the photoconductive layer. Such a weak adhesion brings about exfoliation of the protective layer at the interface between the protective layer and the photoconductive layer when the photoconductor is practically used. It has thus been found that there is still room for improvement in the durability of the photoconductor.